This invention relates generally to injection molding and more particular to an injection molding system having heated nozzles mounted in tandem, the rearward one being fixed in place and the forward one reciprocating between open and closed positions to provide valve gating.
Heated nozzles are well known in the art, and valve gating is usually provided by a valve pin driven by actuating mechanism to reciprocate in a central bore in the nozzle. When molding certain materials it is desirable to provide additional heat to the gate area to facilitate closing of the gate. In the past, this has been done by providing a valve pin of which a portion is formed by a "heat pipe" as disclosed in the applicant's U.S. Pat. No. 4,125,352 entitled "Thermal Valve Pin" which issued Nov. 14, 1978. Another solution to this problem is to improve the conductivity of the valve pin by filling it with copper, as described in the applicant's U.S. Pat. No. 4,406,609 entitled "Valve Pin and Method of Manufacture" which issued Sept. 27, 1983. However, these previous conductive valve pins have the disadvantages that they are relatively costly to make, do not provide accurate temperature control and do not provide sufficient heat as in the gate area for some applications the nozzle. A multi-cavity system in which a number of nozzles having a pointed tip are secured to a common manifold and reciprocated together is shown in the applicant's Canadian patent application Ser. No. 542,182 entitled "Injection Molding Multiple Nozzle Valve Gating System" which was filed July 15, 1987. A single cavity system in which one nozzle is valve gated by rotating a collar is shown in the applicant's Canadian patent application Ser. No. 554,730 entitled "Injection Molding Single Nozzle Valve Gating" filed Dec. 17, 1987. While this arrangement is suitable for certain applications, it is relatively complex and costly to make.
The applicant's copending Canadian patent application Ser. No. 563,982 entitled "Injection Molding System with Nozzles in Tandem" discloses the concept of mounting two heated nozzles axially in tandem. Mounting a piston to drive the forward nozzle with a heated nose portion leading to a tip from a retracted open position to a forward closed position provides a combination of heat and pressure at the gate to form a clean gate when molding with crystaline materials.